I know, you set the pref media.seekToNextFrame.enabled, so it's supposed to work, but https://treeherder.mozilla.org/logviewer.html#?job_id=92558602&repo=try says it doesn't.
You should be able to test locally by just changing /config/milestone.txt from 55a1 to 55.
[Tracking Requested - why for this release]:
Permaorange on the merge to beta, closed tree, delayed b1, sadness.